Microelectronic imagers are used in digital cameras, wireless devices with picture capabilities, and many other applications. Cell phones and Personal Digital Assistants (PDAs), for example, are incorporating microelectronic imagers for capturing and sending pictures. The growth rate of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher pixel counts.
Microelectronic imagers include image sensors that use Charged Coupled Device (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS) systems, or other systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also very popular because they are expected to have low production costs, high yields, and small sizes. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating semiconductor devices. CMOS image sensors, as well as CCD image sensors, are accordingly “packaged” to protect the delicate components and to provide external electrical contacts.
Many imaging devices include semiconductor dies having image sensors located on a front surface of the die to receive incoming radiation. The dies also include external contacts or terminals for electrically coupling the sensors to other circuit elements. In order to prevent the external contacts from interfering with the operation of the sensors or limiting the size and/or location of the sensors, the external contacts at the front surface can be electrically coupled to corresponding external contacts positioned on the opposite side of the die from the sensors (e.g., on the back surface of the die). Through-wafer interconnects (TWIs) are typically used to conduct electrical signals from the sensors, front side external contacts, and associated internal circuitry through the die to the external contacts at the back surface. The TWIs are typically formed by (a) making openings or holes in the die and aligned with the corresponding external contacts, (b) lining the sidewalls of the openings with a dielectric material, and (c) filling the openings with a conductor. Solder balls or other type of electrical couplers can then be attached to the back side external contacts and can be reflowed to couple the die to external devices.